Patrick Hennessey's last book, The Junior Officers' Reading Club, was a testosterone-fuelled firecracker about the exhilaration of fighting, killing and occasionally dying in Afghanistan. It carried you along, even if you were numbed by the relentless military acronyms. But Hennessey, a former officer in the Grenadier Guards, came to see that something was absent, even from his own account. "In all these many soldiers' stories a voice is conspicuously missing, those for whom this is not a war of choice… the Afghan soldiers themselves." His new book attempts to set the record straight.

In 2007 Hennessey was in Helmand, attached to a kandak – a battalion in the Afghan National Army (ANA). His job was to "mentor" the Afghan soldiers, to teach them the "British way" of war, to get them to do everything "correctly and well". The bewildered young men all seemed to be called Muhammad. Far from home, illiterate, unable to read a map, they would wander away from training sessions whenever they got bored. The soldiers from the Nato-led International Security Assistance Force (ISAF) saw them as unreliable, mendacious, sometimes treacherous.

That was only part of the truth. When the kandak went to war, leaders emerged, some of them seasoned soldiers trained in the Soviet Union or in the mujahideen camps in Pakistan. Some had seen more action than the Grenadiers could ever hope to see and knew more about the actual business of fighting in Afghanistan than any foreigner could ever hope to learn. They made a real contribution to the fight.

Two years later Hennessey, now a civilian, returned to Afghanistan to track down his Afghan comrades and to judge whether the ANA was indeed becoming a real army. He found many of the men who had so impressed him before. They were still in Helmand, still fighting for their homes and their lives, long after wave after wave of British mentors had gone home.

By now, western objectives were becoming more modest. We no longer aspired to re-engineer Afghan politics in the democratic mode. Our "strategy" was to withdraw in good order after training an army to secure the country. Billions of dollars were poured in, and Hennessey saw some impressive training facilities. But he was sceptical of the claims made by ISAF spokesmen that the new model army was growing right on target.

Unlike the ANA, the Afghan army which fought alongside the Russians two decades earlier was fully equipped with aircraft, helicopters, artillery and armour. The officers were mostly well trained in the Soviet Union. But many of the weaknesses were the same: disorganisation, high desertion rates, divided loyalties, an erratic performance on the battlefield.

One thing was significantly different. Afghan soldiers rarely turned on their advisers in the Soviet time. The multiplication of "green on blue" incidents today severely corrodes the trust that must exist if "mentoring" is to work.

Even without "mentoring", Afghans can fight well enough if they want to, in an irritatingly un-British way, as the mujahideen and the Taliban have both shown. The regular Afghan army fought successfully for the regime the Soviets left behind them. Things fell apart, not because the army could not fight but because both the army and the regime began to disintegrate.

The Afghan tragedy began in 1978 when a group of politicised Afghan officers brought their men out in support of a bloody communist coup. This time, too, the outcome will be decided by the convolutions of Afghan politics rather than the proficiency of the new model ANA. Who can say how our "strategy" will work then, or whether the soldiers we have trained will point their guns away from President Karzai or towards him?

Hennessey hints at these judgments but does not spell them out. The style that served him before doesn't work so well in this more reflective work. Kandak is useful as a contribution to an important subject but it is not the last word.